The invention relates to an analog switch including two complementary MOS field-effect transitors whose source-drain circuits are located in parallel between the input terminal and the output terminal of the switch, a control signal for controlling the switch being applicable to the gate of the MOS field-effect transistor of the one channel type directly and to the gate of the MOS field-effect transistor of the other channel type via a negator.
An analog switch of this kind, also termed transfer gate in prior art, is known from the German text book on high-frequency circuitry, xe2x80x9cTaschenbuch der Hochfrequenztechnikxe2x80x9d by Meinke and Gundlach, 5th Edition, page M35. FIG. 1 shows the circuit diagram of one such known analog switch.
This known analog switch 10 as shown in FIG. 1 contains two complementary MOS field-effect transitors, namely a P-channel field-effect transistor 10 and an N-channel field-effect transistor 12. Signalling the two field-effect transistors 10 and 12 opposite in phase as needed is achieved by applying the control signal from the control signal terminal 14 used for controlling the switch to the gate of the N-channel MOS field-effect transistor 12 directly and to the gate of the P-channel MOS field-effect transistor 10 via a negator 16. To set the switch into the other active state in which a connection between the input terminal 18 and the output terminal 220 exists a HI potential signal, having namely the potential of the supply voltage Vcc, is applied to the control signal terminal 14, signalling both field-effect transistors 10 and 12 ON; whereas when a LO potential signal, having namely ground potential, is applied to the control signal terminal 14, both field-effect transistors 10 and 12 are signalled OFF in which no connection exists between the input terminal 18 and the output terminal 20.
As dictated by the CMOS technology typical for production of the complementary field-effect transistors 10 and 12 both these field-effect transistors comprise a threshold voltage of approx. 1V. This threshold voltage is the potential which relative to the source of the corresponding transistor needs to be applied to the gate so that this transistor changes from the OFF condition into the ON condition, meaning in other words that in this condition the insulation resistance of the transistor assumes very low values; whereas the insulation resistance of the field-effect transistors becomes very high when the supply voltage Vcc, applied as the control signal to the control signal terminal 14, approaches the sum of the two threshold voltages of the field-effect transistors 10 and 12. When only a relatively low supply voltage is available, this may result in it no longer being possible to signal the field-effect transistors 10 and 12 ON so that the analog switch is no longer able to function as desired. One solution to this problem of the field-effect transistors no longer being signalled ON at low supply voltages would be to make use of field-effect transistors which due to application of another CMOS production process have a substantially lower threshold voltage. Such field-effect transistors can be produced with a threshold voltage in the region of 0V. However, when using such field-effect transistors there is likewise the problem that it may then no longer be possible to signal them totally OFF because the threshold voltage due to its temperature dependency is below the control signal voltage normally put to use for this purpose. The result of this is an undesirably high leakage current flowing in the inactive condition of the switch.
An object of the invention is configuring an analog switch of the aforementioned kind so that it can be reliably switched ON/OFF even at low supply voltages with only a negligable leakage current flowing in the OFF condition.
This object is achieved in accordance with one aspect of the invention by a switch of the aforementioned kind in accordance with the invention in that between the input terminal and output terminal of the switch the series source-drain circuits of three MOS field-effect transistors are inserted, whereby the MOS field-effect transistor located in the middle of the series circuit has a channel type opposite that of the other two MOS field-effect transistors, that the gates of all MOS field-effect transistors of the other channel type are each interconnected and that the threshold voltages of the three MOS field-effect transistors of the series circuit are lower than the threshold voltages of the two complementary MOS field-effect transitors whose source-drain circuits are connected in parallel.
This combination of MOS field-effect transistors having a high threshold voltage and MOS field-effect transistors having a low threshold voltage now achieves that the analog switch can be reliably signalled OPEN (field-effect transistors OFF) and CLOSED (field-effect transistors ON) even when the supply voltage is low.